I will preface my comments by saying I would never knowingly participate ( I mean Own, Moderate or be a Member of) in a site that could read a persons PRIVATE Messages!
I will hopefully answer your question. And hopefully my word is worth something, as this would be a reason for me to LEAVE A SITE.. I will then send a email to LSFS and ask him as our resident technical advisor to confirm or deny...and tell us the tech aspect.
I do not know if our SMF version allows for it or not. I know if it does, I do not know how to use it.
If it does allow for it to be done, I will ask LSFS if it can be deleted. Outside of that. All I can do is give you my word I would never use it, and I would ask Kristin and Adam to do the same. And you would have to base your use of it on those statements!